The isolation and characterization of 24,25-dihydroxycholecalciferol (24,25-dihydroxyvitamin D.sub.3) (M. F. Holick et al., Bicochemistry, II, 4251 [1972]), and the subsequent finding that this second most abundant metabolite of vitamin D.sub.3 (J. L. Omdahl and H. F. DeLuca, Physiological Reviews, 53, 327 [1973]) preferentially stimulates intestinal calcium transport without, at comparable dose levels, mobilizing bone calcium and is biologically synthesized in the kidney at the expense of the production 1.alpha.,25-dihydroxycholecalciferol, the potent, rapid-acting, natural metabolite of vitamin D.sub.3 (J. L. Omdahl and H. F. DeLuca, supra), prompted fairly extensive investigation of the physiological role played by this metabolite (see for example, H. K. Schnoes and H. F. DeLuca, Vitamins and Hormones, 32, 395 [1974]). These investigations have been hampered by the minute amounts of the metabolite available from natural sources, the lack of information concerning the stereochemistry of the metabolic hydroxyl group at C-24 and the effect of the configuration of this group on the biological activity exhibited by 24,25-dihydroxycholecalciferol.
Recently, M. Seki et al. (Chem. Pharm. Bull. [Japan], 21, 2783 [1973]) described the non-stereoselective conversion of demosterol acetate to 24.xi.,25-dihydroxycholesterol by either epoxidation with m-chloroperbenzoic acid followed by hydrolysis or hydroxylation with osmium tetroxide followed by reductive hydrolysis. The diol of undefined stereochemical composition at C-24, as well as the epoxide, were subsequently used for the preparation of 24R,25- and 24S,25 -dihydroxycholecalciferol in a process which involves separation of the epimeric 24,25-epoxides or 24,25-diols followed by the established steps for the conversion of cholesterol derivatives to vitamin D.sub.3 metabolites. Shortly thereafter, H. -Y. Lam et al. (Biochemistry, 12, 4851 [1973]) reported a non-stereoselective synthesis of 24.xi.,25-dihydroxycholecalciferol starting from 3.beta.-acetoxy-27-nor-5-cholesten-25-one and proceeding via 24,25-dihydroxycholesterol. J. Redel et al. (Compt. rend. Acad. Sci. [Paris], 278, 529 [1974]) disclosed a non-stereoselective process for the preparation of the vitamin D.sub.3 metabolite. The latter process started with desmosterol acetate, proceeded through an undetermined mixture of 24R,25- and 24S,25-dihydroxycholesterols and gave an extremely poor (about 1%) yield of an undefined mixture of 24R,25- and 24S,25-dihydroxycholecalciferol. Thus stereospecific synthesis of 24R- and 24S,25-dihydroxycholecalciferol utilizing 24,25-dihydroxycholesterol derivatives of known stereochemistry at C-24 overcoming the deficiencies of the prior art processes and making this important metabolite of vitamin D.sub.3 readily available for biological, clinical and therapeutic use would represent a major contribution to the advancement of the state of the art in the vitamin D field.